1. Field of the Invention
The present invention pertains to a device for the dynamic measurement of the torque of a self-synchronous motor. It also pertains to a device for the servo-control of self-synchronous motor, this control device using this dynamic measurement device.
2. Description of the Prior Art
Self-synchronous motors are motors with a good power/weight ratio, that are ever more frequently being used to make servo-systems, where the tendency is for them to gradually replace the DC motors used up till now.
A self-synchronous motor has the same electromagnetic structure as a standard synchronous motor. Its rotor is generally formed by a permanent magnet while its stator is a polyphase stator, for example a three-phase stator, and consequently has three star-connected armature windings.
In fact, the resemblance with the synchronous motor stops here, for the mode of operation of a self-synchronous motor is quite another matter. It consists, in fact, in supplying the above-mentioned armature windings with currents so that the instantaneous magnetic field resulting from the combination of these currents is always perpendicular to the magnetic moment of the magnet forming the rotor or, in other words, parallel to the APMM (the Axis Perpendicular to the Magnetic Moment of the Magnet).
In the case of a three-phase configuration of the armature windings, it can easily be shown that this condition is achieved by supplying the first, second and third armature windings respectively with currents that are obtained by multiplying the signal, or parameter, for the control of the motor (often called the "controlled current") by a value proportional to respectively cos A, cos (A+120.degree.) and cos (A-120.degree.), where A is the electrical angle between the first armature winding and the above-mentioned APMM.
FIG. 1 appended hereto shows a standard device for the control of a self-synchronous motor, by way of indication. In this figure, the references 1, 2 and 3 respectively represent the three three-phase stator windings of the self-synchronous motor, which are respectively crossed by the instantaneous currents i1, i2, i3.
The electrical angle A between APMM and the winding 1 is measured by a sensor 4, and it is applied respectively to the first three inputs 8, 9, 10 of computation circuits 5, 6, 7 which respectively receive a signal, equal to the control parameter, at their three second inputs, 14, 11, 12. This signal is called a "controlled current", Ic, and is applied to the control input 13 and multiplied by a standardization factor in a multiplier 112.
The circuit 5 computes cos A and multiplies it by the analog value of the signal at 14. The circuit 6 computes cos (A-120.degree.) and multiplies it by the analog value of the signal at 11 and, in the same way, the circuit 7 computes cos (A+120.degree.) and multiplies it by the analog value of the signal at 12. Analog voltages appear at the outputs 15, 16, 17 of these three circuits. These analog voltages are respectively proportional to Ic.cos A, Ic.cos (A-120.degree.) and Ic.cos (A+120.degree.). This actually corresponds to the distribution desired for the three armature currents il, i2 and i3.
These three analog voltages respectively drive three power amplifiers 18, 19 and 20 which themselves give the currents il, i2 and i3 to the three-phase stator windings 1, 2 and 3.
A self-synchronous motor such as this has a great many advantages, obtained at a competitive cost:
high specific power; PA0 high power ratio; PA0 possibility of high speeds PA0 smoothness of the low speeds; PA0 long life; PA0 easy maintenance.
By contrast, since these motors are polyphase AC machines there is no physically expressed signal, as there is in the armature current of the DC motors, which can be considered as a direct image of their electromagnetic torque. Now, in servo-systems, it is particularly useful to have a means available to check the torque at the motor shaft, and this check should be a high quality one.
For example, it is often necessary to:
attenuate the effect of the non-linearities of the amplifiers (including their thresholds);
restrict the maximum value of the torque applied to the load;
achieve the fidelity of the torque transfer function.
Moreover, as the technician does not have the image, in real time, of the electromagnetic torque of the self-synchronous motor, it is not possible for him, during the final adjustment tests in the laboratory, to perform measurements on this value and, in particular, to set up the torque transfer function.
It is all the more essential to resolve the problem of controlling the torque of a self-synchronous motor as these machines, owing to the non-negligible inductance of the stator windings, tend to set up a non-negligible opposing reactive impedance, this reaction being all the greater as the speed is high. This results in a reduction of the electromagnetic torque when the speed rises.
Indeed, since the electromagnetic torque of the self-synchronous motors is naturally equal to the vector product of the "rotating field" vector, created by the polyphase stator windings, and the "magnetic moment" vector of the rotor if, for example, the direction of the induction field created by these windings is adjusted so that, when the motor stops (zero speed), it is parallel to the APMM, the inductive phase shift created when the motor rotates in the stator windings, then necessarily produces a correlative phase shift of the above-mentioned induction field, so that this field stops being parallel to APMM. This causes a reduction in the torque, and this reduction varies as the cosine of the above-mentioned spatial phase shift angle.
Two methods are commonly used to combat this ill effect of the inductance of the armature windings:
A first method consists in achieving an "a priority compensation" which is done in real time as a function of the armature currents and the speed of rotation of the motor. With the value of the inductance of each stator winding being known, the method consists in the application to the stator winding, through the power amplifier associated with it, of an increase in voltage that is all the higher as the rotation speed of the motor is high, so as to compensate for the inductive voltage drops The drawback of this method is the lack of precision and the random phenomena inherent in this type of a priori compensation. Besides, this method in no way resolves the problem of checking the torque of the motor.
A second method, represented schematically by dots and dashes in the above-mentioned FIG. 1, consists in the association, with each power amplifier 18, 19, 20, of a current negative feedback loop, respectively 21 (negative feedback connection 24 and input subtractor 25), 22 (connection 26 and subtractor 27) and 24 (connection 28 and subtractor 29). Thus, a current servo-control is set up in each of the three phases. This other method, which is often combined with the first one, has the following drawbacks:
it does not guarantee perfect symmetry of the three amplification chains;
should there be damage to one of the three amplification chains, the motor suffers a very major deterioration in performance characteristics;
it cannot be used to obtain a real image of the torque, especially during the measurements for the final adjustments.